1. Field of the Invention
The present invention relates to a brake booster system and more particularly to an improved technique for controlling hydraulic boost in response to vehicle operator pedal input.
2. Description of the Related Art
Vehicle braking systems have evolved from simple mechanical brakes to hydraulic brakes and then to power assisted braking systems to reduce a driver's brake pedal effort. Many power assisted braking systems rely on the manifold vacuum created by the engine pistons as they draw air into the engine. A common power brake mechanism such as disclosed on U.S. Pat. No. 5,943,863 employs a housing that is intermediate an operator brake actuating pedal mechanism and a master cylinder. When this brake mechanism is enabled, fluid pressure is supplied to individual wheel brake cylinders or actuators. The housing includes a piston or diaphragm normally exposed on both sides to vehicle manifold vacuum. When the operator actuates the brake pedal, atmospheric pressure is admitted to one side of the piston supplying additional force to the master cylinder piston and enhanced brake line pressure to the individual wheel cylinders. Vehicle braking is still possible in the event of vacuum source failure since operator applied pedal force (without boost) is transmitted to the master cylinder through the boost mechanism.
Hydraulic rather than pneumatic brake boosters have also been proposed. For example, U.S. Pat. No. 4,311,085 utilizes a power steering pump as a source of pressurized hydraulic fluid to provide a power assist during a brake application. This hydraulic brake booster includes a housing which communicates with the pressure source. A control valve within the housing is operable to control the communication of fluid pressure through the housing. In order to operate the control valve, an input member coupled to an operator actuable brake pedal extends into the housing and is movable during a brake application to impart movement to the control valve. Movement of the control valve communicates fluid pressure to a pressure chamber wherein an output member is movable in response to the fluid pressure to effect a power-assisted brake application. The output member is coupled to a conventional brake system master cylinder of which U.S. Pat. No. 4,341,076 may be considered to be typical system.
More recently, electronic control of the valve which applies pressure fluid to the booster has been suggested. U.S. Pat. No. 6,007,160, for example, teaches a method of controlling the operation of an electrohydraulic brake booster to achieve a desired pedal feel. This patent also suggests a power steering pump as a boost pressure source. Brake pedal applied force or the distance the brake pedal travels is monitored by an electronic controller which, in turn, opens or closes a pressure control valve to increase or decrease boost. The method includes sensing brake pedal movement from a fully retracted rest position before a significant resistance to travel of the brake pedal is developed and generating a command pulse that results in application of pressurized hydraulic fluid to the boost piston assembly sufficient to overcome preloaded spring forces and seal friction in the boost piston assembly that would otherwise tend to resist further brake pedal travel. A variable resistance or linear variable displacement transducer are suggested as sensing devices. The method further includes providing a control signal override when a brake pedal “bounce” condition is detected to avoid undesired vehicle braking. A brake pedal bounce condition may occur if the brake pedal is released suddenly so that the brake pedal returns to the fully retracted rest position rapidly enough to bounce off of a mechanical stop at that position and move in the brake apply direction. The patent acknowledges earlier similar systems. Combined hydraulic and pneumatic systems have also been suggested in the prior as disclosed in U.S. Pat. Nos. 4,199,948 and 7,008,024.